1. Field of the Invention
The present invention relates to a control unit for a moving body for controlling the moving body in accordance with a predetermined program, and more particularly to a control unit suitable for controlling a high speed movement of a reciprocating optical system of a copying machine.
2. Description of the Prior Art
In the past, when a position of a moving body is to be controlled, there have been various difficulties in controlling both a acceleration and a velocity. As an example, a speed control of a moving optical system of a copying machine is discussed below.
The moving optical system is usually controlled only in an exposure excursion (i.e. in a forward movement) and not controlled in a reverse or return movement and during stopping. However, when copy speed is to be increased in such an arrangement, an impact at the reversal of the moving optical system influences the exposure process and results in image vibration. If the impact at the reversal is too large, an original sheet illumination lamp which is moved in unison with the moving optical system would be subject to breakage.
Prior art control systems for the moving optical system are described below.
In a first system, two clutches, one for forward movement and the other for backward movement, are used and the activation and the deactivation of those clutches are controlled to effect a reciprocal movement. In the forward movement, the forward clutch is activated to effect a constant velocity drive, and in the backward movement, the forward clutch is deactivated and the backward clutch is activated to effect a backward constant velocity drive.
In such an arrangement, since the forward and backward movements are attained by the activation and the deactivation of the clutches, the acceleration imparted to the movable optical system exceed 10 Gs. The impact due to the forward and backward movements influences the copying machine and it not only results in the vibration of the image but also shortens the lifetime of the copying machine. Further, the noise and the impact at the reversal are not pleasant for a user.
However, this arrangement has an advantage that a scan distance of the movable optical system can be set to a desired distance because the reciprocal movement is controlled by an electrical signal. Thus, it can quickly return the moving optical system depending on a copy size. This system is a main system of the currently available systems but the copy speed is limited to approximately 50 sheets per minute.
In a second system, a direction of rotation of a DC motor is reversed to effect the reciprocal movement of the moving optical system. The forward and backward movements are attained by changing a polarity of a voltage applied to the DC motor, and the constant velocity control is attained by a phase locked loop control.
In this arrangement, while the forward and backward movements of the moving optical system are attained by the on/off control, it has an advantage that the impact is relieved by the inertia of the motor, contrary to the first system described above.
However, during the acceleration and the backward acceleration, the full voltage is applied to the DC motor and no velocity control is effected. Accordingly, an overrun and a time lag at the switching time of the forward and backward movements vary depending on a variation of the friction resistance of a guide rail, the variation of an output and torque characteristic of the DC motor and a fluctuation of the power supply voltage. As a result, copy performance (number of sheets per minute) of the copying machine varies.
Furthermore, while the DC motor used in this arrangement is suitable for the constant velocity rotation, the control in the acceleration and the deceleration is hard to attain for the following reasons.
(a) When an equi-acceleration motion is to be imparted to the moving optical system, a torque output of the DC motor must be kept constant. Accordingly, it is necessary to vary the input voltage Vin in accordance with rotating speed as shown in FIG. 1. Therefore, when the acceleration is to be changed to accelerate or decelerate the moving optical system, complex control is required.
(b) When the rotating speed of the DC motor is low or the direction of rotation of the DC motor is to be reversed, a large current flows as shown in FIG. 2. Therefore, a big power supply is required.
In a third system shown in FIG. 3 (which is similar to that disclosed in FIG. 5 of U.S. Pat. No. 4,120,578), the optical system is reciprocally driven by a cam. The position and the velocity in the forward and backward movements are controlled by a contour curve of the cam 84. Since it can uniquely define the degree of acceleration/deceleration, the position of the reversal and a home position, the smooth control from the constant velocity drive to the backward drive is attained.
Numeral 66 in FIG. 3 denotes a reciprocating optical system, numeral 64 denotes a drive belt, numeral 80 denotes a fixed end of the drive belt, numeral 83 denotes a cam follower and numeral 84 denotes the cam.
However, since the stroke of the moving optical system is uniquely defined by the contour curve of the cam, the moving optical system is full-stroke driven even when a copy size is small. As a result, the copy speed cannot be increased and the lifetime of the copying machine is shortened by the extra physical movement.
Further, this arrangement requires a large scale mechanism and hence a high cost.